The present invention generally relates to rotary gear pumps, and more specifically to rotary gear pump rotor assemblies for use in pumps seeking high efficiency and/or capability to operate within wide temperature ranges.
It is well-known to one skilled in the prior art that rotary gear pump performance is considerably affected by clearances between rotating gear teeth and the adjacent stationary surfaces within a pump, such as the pump casing. To maximize efficiency, it is desirable to minimize these clearances to the greatest extent possible without inducing contact between the rotating gear teeth and the stationary surfaces. Contact may lead to galling, wear, and a variety of additional disadvantageous issues. However, it is further known that such clearances are engineered to maintain operability and a desired efficiency within a well-defined temperature range, and that clearances will change, due to thermal expansion, as temperature increases or decreases. In general, the deformation of the pump components in response to temperature changes may be affected by the material composition and shape of the components, and by other complex factors.
In prior art pumps with a wide operational process temperature range, a geared rotor is typically constructed of a material with a coefficient of thermal expansion similar to that of the casing. This is intended to minimize changes to designed clearances and, consequently, to maintain operability and favorable efficiency throughout the designed temperature range. However, this may restrict the selection of appropriate materials to a subset which may not necessarily include materials that are most appropriate for a particular application, as may be defined by preferred or required mechanical properties, competitive cost, or other factors. Such situations may require a sacrifice in material properties, tightness of clearances affecting pump efficiency, or broadness of process temperature range.
The issue may be characterized by an instance in which it is desirable to employ a thermoplastic material for rotor gear teeth, perhaps to take advantage of the material's resistance to galling and favorable low wear properties. However, the thermoplastic material may have a significantly greater coefficient of thermal expansion than a non-thermoplastic pump casing material. In absence of the present invention, a pump designer must generally chose to forego said advantages of thermoplastic rotor gear teeth, increase design clearances and, consequently, reduce efficiency for a substantial portion of the operational temperature range, or reduce the operational temperature range such that an acceptable efficiency may be achieved throughout.
The present invention addresses shortcomings in prior art gear pumps by providing a way to avoid the aforementioned sacrifice in material properties, pump efficiency, or process temperature range.